Bilge water in ships is often contaminated with oil containing various residues. To prevent polluting the marine environment it is necessary to remove as much of the oil as possible from the bilge water before the bilge water is discharged overboard. The separation process can be particularly difficult where the input to the separator is an emulsion of oil and water. In an oil/water emulsion, fine droplets of oil are dispersed in water, the oil particles being too small to settle out. Prior art separators capable of dealing with oil/water emulsions have employed costly methods.
Prior art oil-water separators are often bulky, inefficient, susceptible to clogging and/or difficult to operate. Some prior art separators employ filters to separate immiscible fluids. Filters in such separators are prone to clogging if the separator does not provide an effective mechanical separation stage prior to the filters to remove oil and its various residues. Consequently, these separators require frequent and labour intensive filter replacements which can render them economically infeasible.
Centrifuge-type separators comprise rotating parts which are susceptible to wear and break downs. Such separators are costly to operate because they require expensive replacement parts.
Hydrocyclones can produce an effluent with an oil content of 50 parts per million (ppm), which fails to meet the 15 ppm maximum imposed by International Maritime Organization (IMO) or United States Coast Guard (USCG) regulations. When two or more hydrocyclones are used in series to reduce the oil content, the recovered oil contains a substantial amount of water which makes the recovered oil difficult to store or reuse. Injecting pure water or chemical substances for improving the performance of prior art hydrocyclones is wasteful. Moreover, hydrocyclones are prone to erosion damage which, in an extreme case, could result in an oil spill.
In some units the separation is carried out downstream from a feed pump. However, the feed pump may emulsify the oil-water mixture that it is delivering to the separator. This makes it difficult to achieve a good separation.
In other prior art designs the entire separation process takes place in a tank prior to the feed pump in order to avoid emulsification. However, in such systems it is almost impossible to remove all of the oil from the separation stages. The remaining oil tends to gradually build up and contaminate the effluent.
Some prior art oil-water separators provide a tank for preliminary separation prior to a feed pump and another tank containing filters on the discharge side of the pump. This arrangement avoids emulsification and makes it possible to keep the filter tank clean by eliminating the coalesced oil at all times. However, this arrangement is wasteful of space which is at a premium aboard ships.